Although Methylmercury (MeHg) is a well-documented developmental neurotoxicant, the precise cellular and molecular mechanisms responsible for MeHg-induced neurotoxicity remain elusive. Prenatal MeHg exposure can lead to impaired neurological development and result in a variety of defects ranging from severe mental retardation, cerebral palsy and blindness to subtle deficiencies in motor function, sensory responses, and learning and memory. Our lab has demonstrated this same spectrum of neurobehavioral abnormalities in adult zebrafish that were exposed during development to MeHg at levels comparable to human exposures. The long-term goal of our laboratory is to identify potential intervention strategies for high-risk human populations by effectively modeling, characterizing and understanding the effects of developmental MeHg-exposure in a zebrafish model system. Appropriate neurobehavioral responses require the functional integration of all nervous system components, while abnormalities strongly suggest alterations in basic anatomy or cellular physiology established during nervous system development. We have shown that zebrafish developmentally exposed to MeHg exhibit neurobehavioral abnormalities including hyperactivity as well as deficits in vision, hearing, and learning and memory. The overall objective of this project is to identify and characterize critical, sensitive neuronal cell populatons altered by developmental MeHg exposure in the context of the whole animal. Our hypothesis is that developmental exposure to MeHg, at concentrations well below that which causes overt toxicity, results in changes to the neuroanatomical structure and gene expression profile of specific and critical cell populations within the developing nervous system. With the proposed study, we will expand upon our zebrafish model of MeHg-induced neurodevelopmental toxicity by using transgenic zebrafish expressing cell-specific fluorescent reporter proteins to identify and characterize the unique populations altered by developmental MeHg exposure. The Specific Aim of this project is to identify specific MeHg-sensitive neuronal populations within the developing nervous system through characterization of neuroanatomical abnormalities and altered gene expression profiles induced by developmental MeHg exposure in the zebrafish model system. This Aim will be achieved through a detailed analysis of specific, fluorescently labeled, cell populations within the developing nervous system using high resolution microscopy, and fluorescence-activated cell sorting (FACS) followed by RNA-seq transcriptome analysis of reporter-positive cells. Understanding the mechanism of MeHg-induced neurotoxicity at a basic biological level as well as in the context of the whole animal is critical in light of te ~15% of American women of childbearing age that have blood mercury concentrations above the level expected to cause developmental deficits in sensitive children.